Accessing values in a 2D array

[vanderr]

Hi everyone. I've recently been asigned a program where we are required to create a 2D array dynamically and then do stuff with it. We are not allowed to "cheat" and use the 1D method. in the constructor of my class i have defined the array as:

//GamePiece is a different user defined class, size is an integer passed to the constructor and handle is the name of the 2D array

handle = new GamePiece*[size];

for(i=0;i<size;i++)
{
handle[i] = new GamePiece[size];
}

In the constructor i have no problem accessing the values in the array with

handle[row][column]

but in any other function in this class i get an unhandled exception, access violation reading location 0x000000.... error. my quesiton is:

is this the correct way to access the values stored in the 2D array within this class or should i overload the () operator?

if i need to overload the () operator, do you have any suggestions on how to go about that? i know how to overload the =, and the <</>>, but i've never had to do the () before. while i'm sure it would be similar to the others, i'd appreciate a little direction if you don't mind. Thanks!

[weaknessforcats]

Actually, there are only one-dimensional arrays in C++. The first index is the number of elements in the array:

int arr[5];        //5 elements. Each element an int
int arr1[5][3];    //5 elements. Each element an array of 3 int
int arr2[5][3][4];  //5 elements. Each element an array of 3 elements that are arrays of 4 int
 

The "dimensions" aare just you telling the compiler about the sizeof the elements.

So, here you have allocated a one dimensional array:

handle = new GamePiece*[size];

This is an array of GamePiece pointers.

Is the GamePiece itself an array??

If not, you can't use the handle[row][column] syntax. All this syntax does is give the compiler instrustions on how to calculate the address you need.

Have you initialized the GamePiece pointers before using them??

An 0x00000000 access violation looks like an attempt to use a null pointer. Big no-no.

There is no indication here that you need to overload the function operator. You are not using functors that I can see.

[vanderr]

the first statement,
handle = new GamePiece*[size];
is declaring an array of gamepiece pointers, the for loop, i thought was making each of those pointers point to an array of gamepiece objects. Is my declaration incorrect? How can i initialize the pointers to fix this error.

we're not allowed to use the [][] syntax for declaring the array, but my teacher said that we could use that syntax for retrieving the values. like i said, i've gotten it all to work in the constrtuctor, but the other functions get the access violation error.

[weaknessforcats]

the first statement,
handle = new GamePiece*[size];
is declaring an array of gamepiece pointers, the for loop, i thought was making each of those pointers point to an array of gamepiece objects. Is my declaration incorrect? How can i initialize the pointers to fix this error.

we're not allowed to use the [][] syntax for declaring the array, but my teacher said that we could use that syntax for retrieving the values. like i said, i've gotten it all to work in the constrtuctor, but the other functions get the access violation error.

You have to use the [][] syntax to allocate an array of arrays.

Let's assume you are doing chess so you have a board of 8 rows and 8 columns. Each square is an bool that is 1 (occupied) or 0 (vacant).

The 2D array is created by:

 bool  (*board)[8] = new bool[8][8];
 

Since the namne of the array (board) is the address of element 0, and element 0 is an array of 8 bool, then you need to declare a pointer to an array of 8 bool.

This is wrong:

bool** board = new bool[8][8];   //won't compile
bool* board  = (bool*)new bool[8][8];  //don't lie to the compiler. Just do bool[64]
bool* board  = new bool[64];     //OK but you lose the 8 rows 8 columns structure
 

That said, this allocation:

handle = new GamePiece*[size];

allocates an array of GamePiece* but the pointers don't point to a GamePiece.

You will need to allocate a GamePiece for each of these pointers:

handle[0] = new GamePiece;
//etc...
 

This is not a 2D array. It is just a 1D array of GamePiece pointers that should be pointing to GamePiece objects.

You will not be able to use handle[row][column] because there no columns.

[vanderr]

thanks anyway, but this code is correct

m_board = new GamePiece*[size];

for(i=0;i<size;i++)
{
m_board[i] = new GamePiece[size];
}

for dynamically allocating a 2D array. i was initializing the pointer to null in the member initialization area, and the executable code wasn't correctly initializing it. once i got rid of the null in the member init, my code has been working fantastically. i still use [][] to access the individual values, but the above is how i declared it.